﻿using System;
using System.Text.RegularExpressions;

class UppercaseTags
{
    static void Main()
    {
        string text = "We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.";

        string result = Regex.Replace(text, "(<upcase>)(.*?)(</upcase>)", m => m.Groups[2].Value.ToUpper());

        Console.WriteLine(result);
    }
}